574 DOLBEAR. [Vou. II. 
length of any particular ray that is produced by the vibration. 
Thus, if the red hydrogen rays have a wave length of 0.000656 
of a millimetre, the frequency of vibration of the hydrogen atom 
that produces it is 457,000,000,000,000 times per second. This 
particular vibration is the 20th harmonic of hydrogen. What 
is of most moment here is to bear in mind that waves of so-called 
light, having definite and constant wave lengths, are the result 
of the heat vibrations of the atoms and of molecules, which must 
therefore be vibrating at a constant rate, not irregularly ; and to 
vibrate thus is one of the fundamental mechanical properties of 
atoms as elastic bodies. 
In order now to see what must follow from such motions, it 
will be best to consider some of the phenomena of vibrations as 
manifested in bodies of visible magnitude, say a tuning-fork. 
If a light body like a pith ball be brought near the prong of a 
vibrating tuning-fork, it will move towards the prong as if the 
latter attracted it, and this is explained as being due to the fact 
that a vibrating body reduces the air pressure adjacent to it. 
How a vibrating body can reduce the pressure of the medium 
about it may be understood by remembering that the pressure 
is proportional to the density. If the vibration lessens the 
density, it lessens the pressure. Let the inner circle of the dia- 
gram, Fig. 2, represent an elastic sphere capa- 
ble of extension to the dimension of the outer 
circle. When it is thus expanded, of course it 
excludes the air from the space it occupies. 
When it contracts again, the air must follow. 
Suppose the contraction should take place at a 
rate greater than the air could move; then 
there would be a vacuum between the surface and the advanc- 
ing air, and that would be the condition all about the sphere ; 
but at whatever rate the contraction took place there would still 
be a partial vacuum next the surface of the sphere, else there 
would be no movement of the air towards it; that is to say, the 
pressure is less next to the sphere than at a distance from it. 
Suppose the sphere to contract and dilate between the limits of 
the lines in the figure, say one hundred times per second; then 
the density would be less between those limits than it would be 
beyond the outer one, with the consequence that the lessened 
density would lessen the pressure upon the sphere in every 
Fic. 2. 
